Author Topic: Starship!  (Read 36997 times)

Offline smartcooky

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Re: Starship!
« Reply #240 on: April 23, 2023, 09:13:09 PM »
Second, (and apologies that I don't remember the terminology), I understand that rocket first stages generally attempt to follow a pre-determined course, and that upper stages have more freedom to match available resources to achievable orbits? Anyway, given that SpaceX seems to have an attitude that Super Heavy is going to have engines fail (and I noticed Manley pointed out the rocket seemed to have an off-angle thrust at one point) is that necessarily the most efficient way to run things? I suppose what I'm asking is, why the two different approaches to guidance between first stage and upper stages? If Super Heavy is going to lose engines, isn't it more fuel efficient to let the rocket decide its own course based on the available engines?

Thank you!


Most rockets use a "yaw program" to tilt the rocket away from the launch tower. This to protect it from any swing arms that failed to move away or a strong gust of wind that might slam the stack back into the launch tower. This technique goes right back to before the Apollo program, and Starship uses this program as well.

In the case of Super Heavy Booster, the entire outer ring of 24 Raptors are fixed - only the inner core of nine engines can gimbal, as this short video demonstrates...


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Offline Peter B

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Re: Starship!
« Reply #241 on: April 24, 2023, 06:31:16 PM »
The restarting engine was probably bad telemetry, or even human error if someone was manually updating that part of the dashboard. The outer ring requires ground infrastructure to spin up turbopumps and deliver gases to the torch igniters.

Cool, thank you.

Quote
I'm not sure what you're asking for the second part, the question seems self contradictory. You're simultaneously asking why the booster didn't have the freedom to change the trajectory to account for available engines, and why it did so. This was very much not the planned trajectory, it was much lower altitude and slower, and burned longer trying to correct things, presumably using the propellant normally reserved for boostback and landing.

Yeah, you're right, I hadn't thought things through.

I think the terms I was looking for were "open loop" and "closed loop" guidance. I understand the Saturn V flew under one system for the first stage, and the other for the remainder of the ascent; and I was wondering if the same was intended for Starship. And having asked that, what's the difference between the two, and did perhaps Starship's guidance system play any role in Starship's atmospheric gymnastics.

Thank you?

Offline smartcooky

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Re: Starship!
« Reply #242 on: April 25, 2023, 07:03:06 AM »
Just a small correction to my post #241

Where I said "In the case of Super Heavy Booster, the entire outer ring of 24 Raptors are fixed - only the inner core of nine engines can gimbal" this was wrong. It related to Booster 4 which actually used only 29 Raptors (20 + 9 not 24 + 9 as I stated)).

However, this launch used Booster 7, and it has a different configuration... the outer ring has 20 fixed engines, and the inner core of 13 can gimbal.

The video related to Booster 4
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Offline cjameshuff

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Re: Starship!
« Reply #243 on: May 14, 2023, 03:08:40 PM »
Strangely, I haven't heard any complaints that they didn't put a "useful" payload on this Starship flight...

Offline smartcooky

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Re: Starship!
« Reply #244 on: May 14, 2023, 11:53:14 PM »
Strangely, I haven't heard any complaints that they didn't put a "useful" payload on this Starship flight...

I promise you that if the launch had successfully achieved its target of a suborbital flight, the "20/20 Hindsight" brigade would be complaining.
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Offline Dalhousie

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Re: Starship!
« Reply #245 on: May 15, 2023, 01:30:13 AM »
Strangely, I haven't heard any complaints that they didn't put a "useful" payload on this Starship flight...

I promise you that if the launch had successfully achieved its target of a suborbital flight, the "20/20 Hindsight" brigade would be complaining.

I doubt that very much.  The stated goals were clear, reasonable, and there was no stunt involved, unlike the risible car in space of the 1st FH flight.




Offline JayUtah

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Re: Starship!
« Reply #246 on: May 16, 2023, 02:35:38 PM »
I think the terms I was looking for were "open loop" and "closed loop" guidance. I understand the Saturn V flew under one system for the first stage, and the other for the remainder of the ascent; and I was wondering if the same was intended for Starship. And having asked that, what's the difference between the two...

Closed-loop control incorporates measurements of the property the controller is meant to control and adjusts the control input accordingly. Open-loop control applies the control according to invariant properties of the controller program.

Open-loop control is like a sprinkler system without any measurement of soil moisture content. It will dutifully come on at 4:00 AM and apply 15 minutes' worth of water control even if your entire garden is already under water. Closed-loop control would adjust the amount of watering based on how much water is already in the soil.

Open-loop guidance says to fly the rocket in Attitude A for so many seconds, then Attitude B for so many other seconds, and so forth—all regardless of whether the rocket is in the right place and moving in the right direction. Closed-loop guidance adjusts the desired rocket attitude based (in part) on whether the rocket is on the desired trajectory and whether that trajectory is getting you where you want to go. The LM ascent program was also open loop. It was the CSM's job to swoop down and find the LM ascent stage, wherever in orbit it ended up as a result.
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Offline smartcooky

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Re: Starship!
« Reply #247 on: May 28, 2023, 07:14:14 AM »
I would like some of our "rocket scientists" here have a look at this video and offer their opinions. It is an animated render and commentary giving a brief explanation of how the water-cooled steel plate system is supposed to work. Its from Alpha Tech's YouTube channel, with the computer rendering by Ryan Hansen. Credit to both of them for this excellent presentation.

Note: right-click on the link and choose to play it in a new tab. It will play a short (1m 41s) video in Dropbox

https://www.dropbox.com/s/1zi550dn801f41m/Starship%20Deluge.mp4?raw=1

As most of you already know, I am an aeronautical engineer (retired), but not an aerospace engineer. While the two disciplines share a lot of common ground, there are aspects of "rocket science" that are outside my pay-grade.
 
In particular, there are some statements made that I am doubting the accuracy of. I would appreciate any comments on some of these technical details.

1. The claimed "established temperature gradient" between the rocket exhaust at the point it leaves the ending bells and the water cooled plate
2. "In the centre of the plate, the water pressure is higher than the exhaust pressure"
3. "The exhaust never touches the plate"
4. If the water flow is maintained at a high enough rate, they can keep the temperature at the plate below 1000°C
5. In the real world, the plate never sees more than a couple of hundred degrees C



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Offline bknight

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Re: Starship!
« Reply #248 on: May 28, 2023, 09:43:32 AM »
I would like some of our "rocket scientists" here have a look at this video and offer their opinions. It is an animated render and commentary giving a brief explanation of how the water-cooled steel plate system is supposed to work. Its from Alpha Tech's YouTube channel, with the computer rendering by Ryan Hansen. Credit to both of them for this excellent presentation.

Note: right-click on the link and choose to play it in a new tab. It will play a short (1m 41s) video in Dropbox

https://www.dropbox.com/s/1zi550dn801f41m/Starship%20Deluge.mp4?raw=1

As most of you already know, I am an aeronautical engineer (retired), but not an aerospace engineer. While the two disciplines share a lot of common ground, there are aspects of "rocket science" that are outside my pay-grade.
 
In particular, there are some statements made that I am doubting the accuracy of. I would appreciate any comments on some of these technical details.

1. The claimed "established temperature gradient" between the rocket exhaust at the point it leaves the ending bells and the water cooled plate
2. "In the centre of the plate, the water pressure is higher than the exhaust pressure"
3. "The exhaust never touches the plate"
4. If the water flow is maintained at a high enough rate, they can keep the temperature at the plate below 1000°C
5. In the real world, the plate never sees more than a couple of hundred degrees C

I'm not an aerospace engineer either but have some experience with high flow water rates/pressures.
It seems to me that 2,3,4 are dictated by a high enough rate to prove all the cases correct.  Since I don't know the temperature of the exhaust nor the profile of the exhaust i can't peak of those parameters.  I suspect they have either modelled or calculated the exhaust from engine bell to tip of exhaust, so they know the temperature/pressure regime that the water flow will experience and then back calculate how much flow is needed to offset the exhaust.  I don't know the diameter of the holes nor the number of the holes, but this two would be a somewhat easy calculation in fluid flow requirements.
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